Fuel oil treatment

ABSTRACT

PCT No. PCT/EP92/01481 Sec. 371 Date Feb. 28, 1994 Sec. 102(e) Date Feb. 28, 1994 PCT Filed Jul. 1, 1992 PCT Pub. No. WO93/01260 PCT Pub. Date Jan. 21, 1993.An additive comprising an oil-soluble stable free radical, such as a nitroxide, or a precursor therefor, is used in a fuel oil to reduce, on combustion of the fuel oil, one or more of particulate emissions, hydrocarbon emissions, carbon monoxide emissions, and oxides of nitrogen emissions.

This invention relates to the use of additives in fuel oils to reduceemissions on combustion of the fuel oil and to increase engine powerwhen used in an internal combustion engine.

Although modern internal combustion engines are highly efficient andgive almost complete combustion of the hydrocarbon fuel used, the slightreduction from total efficiency leads to the formation of black smoke, aproportion of which is particulate carbon and other products ofincomplete combustion. Apart from the smoke being unpleasant to breatheand unsightly, the carbon particles may have absorbed in thempolynuclear hydrocarbons, which also result from incomplete combustion,some of which are known carcinogens.

Furthermore, internal combustion engines give rise to gaseous emissionson combustion of fuel therein, examples of such emissions being one ormore hydrocarbons, carbon monoxide, and oxides of nitrogen and whichexamples are noxious and undesirable.

U.S. Pat. No. 4,398,505 describes diesel fuel compositions includingN,N-disubstituted organic nitroxides, which are organic free radicals,in amounts effective to increase the cetane number of the fuel. Suchamounts are stated to ordinarily be in the range from about 0.01 to 5weight percent based on the total weight of the resulting fuel,preferably from about 0.1 to about 3 weight percent. However, U.S. Pat.No. 4,398,505 neither discloses nor suggests that organic free radicalssuch as those disclosed therein are capable of reducing emissions oncombustion of the fuel. This effectiveness has now been found, accordingto this invention. Thus, the invention provides in one aspect the use asan additive in a fuel oil to reduce, on combustion of the fuel oil, oneor more of particulate emissions, hydrocarbon emissions, carbon monoxideemissions, and oxides of nitrogen emissions, the additive comprising anoil-soluble stable free radical or a precursor therefor.

Furthermore, U.S. Pat. No. 4,398,505 neither discloses nor suggests theuse of organic free radicals in amounts less than those effective toincrease the cetane number of the fuel. It has now been found, accordingto this invention, that such amounts are effective in reducing emissionson combustion of fuels. Thus, in a second aspect, the invention providesa composition comprising a major proportion of a fuel oil and, dissolvedtherein, a minor proportion of an additive comprising an oil-solublefree radical or a precursor therefor, the minor proportion of theadditive being such that it is incapable of increasing the cetane numberof the fuel oil; and in a third aspect, the invention provides a methodfor operating a diesel engine to reduce one or more of particulateemissions, hydrocarbon emissions, carbon monoxide emissions, and oxidesof nitrogen emissions in operation of the engine, which method comprisesoperating the engine using a composition comprising a major proportionof a fuel oil and a minor proportion of an additive comprising anoil-soluble stable free radical or a precursor therefor, the minorproportion of the additive being such that it is incapable of increasingthe cetane number of the fuel oil.

Moreover, U.S. Pat. No. 4,398,505 neither discloses nor suggests usingorganic free radicals in additive combinations with co-additives. Suchcombinations have been found to be surprisingly beneficial according tothe invention and, in a fourth aspect, the invention provides anadditive combination comprising an oil-soluble stable free radical or aprecursor therefor and one or more co-additives as defined hereinafterin this specification. Such a combination may be included in a fuel oilcomposition or in a concentrate for addition to a fuel oil.

It is noteworthy in the present invention that the organic free radicalsor precursors therefor, when incorporated in a diesel, heating or jetfuel, may reduce the emissions of particulates even in the absence ofinjector deposits. Thus the reduction in particulate emission achievedby the present invention may result directly on combustion of a fuelcontaining the free radical or precursor, compared with the emissionsresulting from combustion, in the same combustion chamber with the sameconditions upstream of the combustion chamber, of fuel not containingthe free radical or precursor but otherwise identical. Such reduction isherein referred to as the "direct" reduction.

While the applicants do not wish to be bound by any theory, it isbelieved that under given conditions (which include any deposits presentin injectors or elsewhere upstream of the combustion chamber) thepresence of the free radical or precursor in the fuel, or in thefuel/air mixture, in the combustion chamber results in an improvement inthe quality of combustion, as measured by completeness of oxidation.This improvement may in turn be the result of a change in the physicalproperties of the fuel, or the fuel/air mixture, e.g. the surfacetension of the fuel, resulting in improved mixing and reduced soot andsmoke formation. The reference above to the presence of the free radicalor precursor includes the presence of a reaction product of the freeradical or precursor with a component of the fuel, the reaction havingtaken place either before entry into the combustion chamber or withinthe combustion chamber prior to combustion.

The features of the invention will now be discussed in further detail.

ADDITIVE

Stable free radicals or precursors therefor such as can be used in thisinvention are described in U.S. Pat. No. 4,670,131, which describestheir use in controlling fouling in equipment for processing and storinghydrocarbon compositions containing unsaturated hydrocarbons.

By "stable free radical" in this specification is meant a free radicalthat can be prepared by conventional chemical methods and that can existlong enough under the conditions of combustion of a fuel oil for it tobe able to influence combustion to reduce one or more of theabove-mentioned emissions. Generally, the free radical should be capableof existing under such conditions for longer than the time of thecombustions reactions occurring during combustion.

By "precursor" in this specification is meant a chemical species that isconvertible to the stable free radical during the combustion.

Examples of stable free radicals for use in this invention are:nitroxides, hindered phenoxys, hydrazyls, stabilised hydrocarbonradicals, and polyradicals of the afore-mentioned free radicals.Examples of precursors for stable free radicals are: nitrones, nitrosos,thioketones, benzoquinones, and hydroxylamines.

In this invention, preferred nitroxides are of the formula: ##STR1##wherein each of R¹⁷, R¹² and R¹³ and R¹⁴ is a hydrocarbyl group orhetero-atom substituted hydrocarbyl group having 1 to 200 carbon atoms,R¹⁵ and R¹⁶ (a) each being a hydrocarbyl group having 1 to 200 carbonatoms, or a substituted hydrocarbyl group having 1 to 200 carbon atomswherein the substituent is halogen, cyano, --CONH₂, --SC₆ H₅,--S--COCH₃, --OCOCH₃, --OCOC₂ H₅, carbonyl, alkenyl wherein the doublebond is not conjugated with the nitroxide moiety, or --COOR wherein R ofthe --COOR group is alkyl or aryl, or (b) together forming part of aring that contains carbon atoms and up to two hetero-atoms of O, N or S,optionally linked to a side chain for enhancing the solubility of thefree radical in a fuel oil.

"Hydrocarbyl" means an organic moiety composed of carbon and hydrogenwhich may be aliphatic, including alicyclic; aromatic; or anycombination thereof, e.g. aralkyl. For example, the moiety may be apolymonoolefin group such as polyisobutylene. Preferably, one or more ofthe hydrocarbyl groups is an alkyl group having 1 to 15 carbon atoms.When hetero atom substituted, the hetero atom may be nitrogen, oxygen orsulphur; there may be one or more such hetero atoms.

In the above formula, it is preferred that the ring, where present,contains 4 to 8 such as 4 or 5 carbon atoms. Examples of the optionalside chain are groups of the same definition of R¹⁵ and R¹⁶ herein whichmay optionally be linked to the ring via a functional group, examples ofwhich being --NH--; --S--; --O--; --CO--; --CS--; --CO--NH--;--CS--NH--; --O--CO--NH--; --O--CS--NH--; --S--CO--NH--; --S--CS--NH--;--O--O--; >C═N--; and >C═N--O--.

The selection of such a functional group may be determined primarily bymanufacturing ease; otherwise its selection may not be critical.

Preferably, each of R¹⁷, R¹², R¹³ and R¹⁴ is a methyl, ethyl or propylgroup. Also, each of R¹⁵ and R¹⁶ may be a methyl, ethyl or propyl group.A specific example of a nitroxide for use in this invention is:4-hydroxy-2,2,6,6-tetramethylpiperindinyloxy.

The additive may, when the invention is the use or the composition, bepresent in the fuel oil in a proportion in the range of from 1 to 1000ppm of active ingredient by weight based on the weight of the fuel oil.Preferably the proportion is in the range of 4 to 500 ppm. Morepreferably, and as stated above in respect of the fuel composition andmethod aspects of this invention, the proportion is insufficient toinfluence or change the cetane number of the fuel. It is, for example,less than 100 ppm such as less than 80 ppm, notably 1 to 50 ppm.

CO-ADDITIVES

The additives of the invention may be used in combination with one ormore co-additives. Particular noteworthy co-additives are the ashlessdispersants which are described in numerous patent specifications andwhich are additives that leave little or no metal-containing residue oncombustion. Many classes are known such as described in EP-A-0 482 253and to which attention is directed for further details thereof. Examplesof co-additives, which include examples of ashless dispersants, are asfollows:

(i) Macrocyclic Compound

Such a compound is an oil soluble compound of the formula ##STR2## ormixtures of two or more such compounds, wherein R¹, R² and R³ may be thesame or different and are independently hydrogen or a hydrocarbylsubstituent having from 2 to 600 carbon atoms, or a keto, hydroxy,nitro, cyano, or alkoxy derivative thereof, provided that at least oneof R¹, R² and R³ is a hydrocarbyl substituent having from 2 to 600carbon atoms or said derivative thereof, or wherein R¹ and R² togetherform a hydrocarbylene substituent having 4 to 600 carbon atoms or aketo, hydroxy, nitro, cyano or alkoxy derivative thereof, provided thatR¹ and R² together with the carbon atom which forms the C-R1 bond withR¹ and the nitrogen atom which forms the N-R² bond with R² form a ringhaving at least 5 members, wherein Z represents

    --R.sup.10 [NR.sup.11 (R.sup.10)].sub.e --

or

    --[R.sup.10 R.sup.11 N].sub.f R.sup.10 [NR.sup.11 R.sup.10 ].sub.g --

wherein each R¹⁰, which may be the same or different, represents analkylene group having from 1 to 5 carbon atoms in its chain, R¹¹represents a hydrogen atom or a hydrocarbyl group, and e is from 0 to 6,f is from 1 to 4, g is from 1 to 4, provided that f+g is at most 5, eachR⁴ is independently H or an alkyl group having up to 5 carbon atoms, R⁵is an alkylene group having up to 6 carbon atoms in the chain,optionally substituted by one or more hydrocarbyl groups having up to 10carbon atoms, an acyl group having from 2 to 10 carbon atoms, or a keto,hydroxy, nitro, cyano or alkoxy derivative of a hydrocarbyl group havingfrom 1 to 10 carbon atoms or of an acyl group having from 2 to 10 carbonatoms, R6 is a hydrocarbyl substituent having from 2 to 600 carbon atomsor said derivative thereof, b is from 1 to 6, c is from 1 to 6 and d isfrom 0 to 12.

For example, the compounds of formula (I) may be ##STR3## wherein R⁷ isa hydrogen or a hydrocarbyl substituent having from 1 to 600 carbonatoms, R⁸ is hydrogen or a C₁ to C₁₂ hydrocarbyl substituent, and ifthere is more than one R⁸ in a compound, they may be the same ordifferent, R⁹ is a hydrocarbylene substituent having from 2 to 600carbon atoms, two of which carbon atoms are bonded to the a-carbon atomsof the succinic anhydride based ring, X₁ represents hydrogen or an alkylgroup having from 1 to 12 carbon atoms, X₂ represents hydrogen, an alkylgroup having from 1 to 12 carbon atoms, a hydroxy group, or an alkoxygroup, the alkoxy group having from 1 to 12 carbon atoms, or X₁ and X₂may together represent an oxygen (or sulphur) atom, and a is 1 to 20.

Macrocyclic compounds such as the above are described in U.S. Pat. No.4,637,886 and U.S. Pat. No. 4,880,923. When the invention is the use orthe composition, the macrocyclic compound, if present, is advantageouslyin a proportion in the range of from 5 to 20,000 ppm of activeingredient by weight based on the weight of the fuel oil, preferablyfrom 10 to 5,000, more preferably from 50 to 3,000.

(ii) Cetane Improvers

It has been found that using a cetane improver in combination with theadditive of the invention and optionally with a macrocyclic compound asdescribed above may give rise to operational benefit.

Preferred cetane improvers are organic nitrates; there may also be used,for example, substituted triazoles and tetrazoles, for example thosedescribed in European Patent Application No 230783. Preferred organicnitrates are nitrate esters containing aliphatic or cycloaliphaticgroups with up to 30 carbon atoms, preferably saturated groups, andpreferably with up to 12 carbon atoms. As examples of such nitrates,there may be mentioned methyl, ethyl, propyl, isopropyl, butyl, amyl,hexyl, heptyl, octyl, iso-octyl, 2-ethylhexyl, nonyl, decyl, allyl,cyclopentyl, cyclohexyl, methylcyclohexyl, cyclododecyl, 2-ethoxyethyl,and 2-(2-ethoxyethoxy) ethyl nitrates.

When the invention is the use or the composition, the cetane improver isadvantageously present in the fuel in a proportion in the range of from5 to 10,000 ppm of active ingredient by weight based on the weight ofthe fuel, preferably from 50 to 5,000, more preferably from 100 to2,000.

(iii) Polymer of Monoolefin

The presence of a polymer of a C₂ to C₆ monoolefin, the polymer having anumber average molecular weight of less than about 500, may also beadvantageous. Such a polymer may, for example, be a homo- or copolymerof ethylene, propylene, butylene (1- or 2-), pentylene or isobutylene,polyisobutylene being preferred. When it is a copolymer, it may be acopolymer of two or more of the specified monomers, or a copolymer ofone or more of the specified monomers with a copolymerisable unsaturatedmonomer. Further, it may be a block or a random copolymer.

The number average molecular weight is as measured by Gel PermeationChromatography (GPC). Preferably, it is in the range of 300 to 500, morepreferably 350 to 450. The polymer may, for example, have a kinematicviscosity at 100° C. in the range of 1 to 20 cSt, preferably 4 to 16cSt, more preferably 8 to 12 cSt.

The polymer may be made, for example, by catalysed polymerisation usingcationic catalyst systems described in the art such as AlCl₃ /H₂ O;AlCl₃ /HCl; Et AlCl₂ /HCl; BF₃ ; or Ziegler-Natta type catalysts.

When the invention is the use or the composition, the polymer isadvantageously present in the fuel in a proportion in the range of from5 to 10,000 ppm of active ingredient by weight based on the weight ofthe fuel, preferably from 50 to 5,000, more preferably from 100 to2,000.

(iv) Other Additive Components

In the practice of this invention, the additive or co-additives, ifpresent, may be used in combination with one or more other additives,for example additives providing particular properties such asdispersants, for example hydrocarbyl-substituted succinimides orsuccinamides and hydrocarbylpolyamines; metallic-based combustionimprovers such as ferrocene; corrosion inhibitors; anti-oxidants such asamine-formaldehyde products; anti-foams; reodorants; anti-wear agents;flow improvers; wax antisettling additives or other operabilityimprovers; and cloud point depressants.

Examples of the above other additive components are known in the art.Such other additives may, for example, be present in the fuel oil in aproportion in the range of 5 to 500 ppm (weight:weight).

PROPORTIONS

Where the additive of the invention is used in combination with one ormore co-additives, the relative proportion of the additives to oneanother may, for example, be in the weight:weight ratio of 500:1 to1:500 such as 10:1 to 1:10.

FUEL OIL

The fuel oils that can be used are petroleum compositions comprisinghydrocarbons such as straight chain paraffins, branched chain paraffins,olefins, aromatic hydrocarbons, and naphenic hydrocarbons, andhetero-atom containing derivatives of the above. The components of thefuel oil can be derived by any of the conventional refining and blendingprocesses. Synthetic fuels are also included.

The fuel oils can be middle distillate fuel oils such as diesel fuel,aviation fuel, kerosene, fuel oil, jet fuel and heating oil. Generally,suitable distillate fuels are those boiling in the range of 120° C. to500° C. (ASTM D-86). A heating oil may have a specification with a 10%distillation point no higher than 226° C., a 50% distillation point nohigher than 282° C., and a 90% distillation point of at least 282° C.and no higher than about 338° C. to 343° C. or possibly 357° C. Heatingoils are preferably a blend of virgin distillate, e.g. gas oil ornaphtha, and cracked distillate, e.g. catalytic cycle stock. A dieselfuel may have a specification that includes a minimum flash point of 38°C. and a 90% distillation point between 282° C. and 338° C. (see ASTMDesignations D-396 and D-975).

The additive and co-additives, if to be provided, may be added to thefuel oil as a mixture or separately in any order using conventional fueladditive injection methods, e.g. in the form of a concentrate. In aconcentrate, the additive(s) may be dissolved in the solvent at aconcentration within wide limits according to needs and restrictions,for example from 20 to 90, such as 30 to 80, per cent (weight:weight).Examples of such solvents are hydrocarbons or oxygen-containinghydrocarbons such as kerosene, aromatic naphthas, and minerallubricating oils.

EXAMPLES

The invention will now be particularly described by way of example onlyas follows:

Example 1

An additive of the invention was tested in a truck engine to determineits effect on hydrocarbon, carbon monoxide, oxides of nitrogen, andparticulate emissions. The engine used was a 6 liter 6 cylinder 4 strokenaturally aspirated DI truck engine.

The fuel used had the following characteristics:

Cloud point: -6° C.

Pour point: -27° C.

Cetane Number (CFR): 51.3

    ______________________________________                                        Distillation Characteristics:                                                 Volume % Off     Temp (°C.)                                            ______________________________________                                        Initial Boiling Point                                                                          148                                                           5               194                                                          10               209                                                          20               229                                                          30               248                                                          40               263                                                          50               275                                                          60               286                                                          70               298                                                          80               312                                                          90               331                                                          95               345                                                          Final Boiling Point                                                                            367                                                          ______________________________________                                    

The tests were run in the following manner according to a standard ECER49 cycle:

1. The engine was warmed up over a period of 90 minutes to full speedand full load.

2. A stabilising test was run using untreated fuel.

3. A test was run on the untreated fuel and emission data werecollected.

4. A test was run on fuel treated with 500 ppm (weight:weight) of anadditive comprising 4-hydroxy-2,2,6,6-tetramethylpiperindinyloxy.

The results are summarised in Table 1 below:

                  TABLE 1                                                         ______________________________________                                                   HC   CO       NO.sub.x                                                                             Particulates                                  ______________________________________                                        Untreated Fuel                                                                             1.195  5.306    8.520                                                                              1.097                                       Treated Fuel 0.958  3.816    9.371                                                                              0.784                                       ______________________________________                                    

All figures represent g/kWh of the indicated emission, HC beinghydrocarbons, CO being carbon monoxide, NO being oxides of nitrogen, andPARTICULATES being particulate matter collected via a conventionaldilution tunnel.

The above results show that the additive reduced each of thehydrocarbon, carbon monoxide and particulates emissions.

Example 2

The following formulations were used:

    ______________________________________                                        Additive Formulation A (Comparison)                                           ______________________________________                                        Macrocycle**              150                                                 Cetane Improver (Octyl Nitrate)                                                                         750                                                 Polymer (polyisobutene; M.sub.N = 450)                                                                  500                                                 *Anti-Foam (alkoxy-substituted poly-dimethy                                                               12.5                                              poly-siloxane)                                                                *Solvent (aromatic hydrocarbon)                                                                           52.5                                              *Anti-Rust (alkoxy-substituted alkylphenol)                                                              20                                                 *Demulsifier               15                                                 Total:                    1500                                                ______________________________________                                         *A cyclic acyl amidine compound of the type described in U.S. Pat. Nos.       4,637,886 and 4,880,923.                                                 

    ______________________________________                                        Additive Formulation B                                                        ______________________________________                                        Macrocycle (as used in Additive Formulation A)                                                           150                                                Cetane Improver (Octyl Nitrate)                                                                          750                                                Polymer (polyisobutene; M.sub.N = 450)                                                                   500                                                Nitroxide (as in Example 1)                                                                               50                                                Solvent (aromatic hydrocarbon)                                                                           1050                                               Total:                     2500                                               ______________________________________                                    

In the above formulations, the numbers indicate quantities in ppm(weight:weight) present in the fuel when tested The components markedwith a single asterisk are believed to have no significant effect onemissions.

The above additive formulations (A and B) were each tested in a carengine to determine their effects on hydrocarbon, carbon monoxide,oxides of nitrogen, and particulate emissions. The engine used was a 1.7liter naturally aspirated IDI passenger car engine.

The fuel used had the same characteristics as the fuel used in Example1.

The tests were carried out in accordance with a standard ECE 15.04+EUDCregistered cycle and were carried out using untreated fuel and fuelscontaining each of additives A and B.

The results are summarised in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                    HC    CO     NO.sub.x Particulates                                ______________________________________                                        Untreated Fuel                                                                              1.61    8.32    9.92  1.40                                      Fuel with Additive A                                                                        3.35    8.48   10.3   1.20                                      Fuel with Additive B                                                                        1.08    7.49   10.6   1.11                                      ______________________________________                                    

The figures represent quantities as described for Table 1.

The results show that Additive B (of the invention) reduces emissions ofhydrocarbons, carbon monoxide, and particulates when compared with theuntreated fuel and with Additive A (comparison).

We claim:
 1. A composition comprising a major proportion of a fuel oiland a minor proportion of an additive comprising an oil-soluble stablefree radical or a precursor therefor that is convertible to anoil-soluble stable free radical under combustion of said composition,the minor proportion of additive being less than 100 ppm such that it isincapable of increasing the cetane number of the fuel oil.
 2. Thecomposition of claim 1 wherein the nitroxide is of the formula: ##STR4##wherein each of R¹⁷, R¹² and R¹³ and R¹⁴ is a hydrocarbyl group orhetero-atom substituted hydrocarbyl group having 1 to 200 carbon atoms,R¹⁵ and R¹⁶ (a) each being a hydrocarbyl group having 1 to 200 carbonatoms, or a substituted hydrocarbyl group having 1 to 200 carbon atomswherein the substituent is halogen, cyano, --CONH₂, SC₆ H₅, --S--COCH₃,--OCOCH₃, --OCOC₂ H₅, carbonyl, alkenyl wherein the double bond is notconjugated with the nitroxide moiety, or --COOR wherein R of the --COORgroup is alkyl or aryl, or (b) together forming part of a ring thatcontains carbon atoms and up to two hetero-atoms of O, N or S,optionally linked to a side chain for enhancing the solubility of thefree radical in a fuel oil.
 3. The composition of claim 2 wherein one ormore of the hydrocarbyl groups is an alkyl group having 1 to 15 carbonatoms.
 4. The composition of claim 2 or claim 3 wherein the ringcontains 4 or 5 carbon atoms.
 5. The composition of any of claim 4wherein each of R¹⁷, R¹², R¹³ and R¹⁴ is a methyl, ethyl or propylgroup.
 6. The composition of any of claim 5 wherein each of R¹⁵ and R¹⁶is a methyl, ethyl or propyl group.
 7. The composition of any of claim 5wherein the nitroxide is 4-hydroxy-2,2,6,6-tetramethylpiperindinyloxy.8. The composition of claim 1 wherein the fuel oil is a middledistillate petroleum fuel oil.
 9. The composition of claim 1 wherein theproportion is in the range of 10 to 80 ppm.
 10. The composition of claim1 wherein the additive is present in combination with one or moreco-additives.
 11. The composition of claim 10 wherein the or one of theco-additives is an oil soluble compound of the formula ##STR5## ormixtures of two or more such compounds, wherein R¹, R² and R³ may be thesame or different and are independently hydrogen or a hydrocarbylsubstituent having from 2 to 600 carbon atoms, or a keto, hydroxy,nitro, cyano, or alkoxy derivative thereof, provided that at least oneof R¹, R² and R³ is a hydrocarbyl substituent having from 2 to 600carbon atoms or said derivative thereof, or wherein R¹ and R² togetherform a hydrocarbylene substituent having 4 to 600 carbon atoms or aketo, hydroxy, nitro, cyano or alkoxy derivative thereof, provided thatR¹ and R² together with the carbon atom which forms the C-R1 bond withR¹ and the nitrogen atom which forms the N-R² bond with R² form a ringhaving at least 5 members, wherein Z represents

    --R.sup.10 [NR.sup.11 (R.sup.10)].sub.e --

or

    --[R.sup.10 R.sup.11 N].sub.f R.sup.10 [NR.sup.11 R.sup.10 ].sub.g --

wherein each R¹⁰, which may be the same or different, represents analkylene group having from 1 to 5 carbon atoms in its chain, R¹¹represents a hydrogen atom or a hydrocarbyl group, and e is from 0 to 6,f is from 1 to 4, g is from 1 to 4, provided that f+g is at most 5, eachR⁴ is independently H or an alkyl group having up to 5 carbon atoms, R⁵is an alkylene group having up to 6 carbon atoms in the chain,optionally substituted by one or more hydrocarbyl groups having up to 10carbon atoms, an acyl group having from 2 to 10 carbon atoms, or a keto,hydroxy, nitro, cyano or alkoxy derivative of a hydrocarbyl group havingfrom 1 to 10 carbon atoms or of an acyl group having from 2 to 10 carbonatoms, R6 is a hydrocarbyl substituent having from 2 to 600 carbon atomsor said derivative thereof, b is from 1 to 6, c is from 1 to 6 and d isfrom 0 to
 12. 12. The composition of claim 11 wherein the compound offormula (I) is ##STR6## wherein R⁷ is a hydrogen or a hydrocarbylsubstituent having from 1 to 600 carbon atoms, R⁸ is hydrogen or a C₁ toC₁₂ hydrocarbyl substituent, and if there is more than one R⁸ in acompound, they may be the same or different, R⁹ is a hydrocarbylenesubstituent having from 2 to 600 carbon atoms, two of which carbon atomsare bonded to the a-carbon atoms of the succinic anhydride based ring,X₁ represents hydrogen or an alkyl group having from 1 to 12 carbonatoms, X₂ represents hydrogen, an alkyl group having from 1 to 12 carbonatoms, a hydroxy group, or an alkoxy group, the alkoxy group having from1 to 12 carbon atoms, or X₁ and X₂ may together represent an oxygen (orsulphur) atom, and a is 1 to
 20. 13. The composition of claim 12 whereinone or both of R⁷ and R⁹ is or is derived from a C₂ to C₅ olefinpolymer.
 14. The composition of claim 13 wherein the polymer ispolyisobutylene.
 15. The composition of any of one claims 11 to 14wherein the co-additive is present in the fuel oil in a proportion-inthe range of from 50 to 20,000 ppm of active ingredient by weight basedon the weight of the fuel oil.
 16. The composition of claim 15 whereinthe proportion is in the range of 10 to 500 ppm.
 17. The composition ofclaim 10 wherein the or one of the co-additives is a cetane improver.18. The composition of claim 17 wherein the cetane improver is analiphatic or cycloaliphatic nitrate.
 19. The composition of claim 18wherein the nitrate is an alkyl or cycloalkyl nitrate containing up to30 carbon atoms.
 20. The composition of any one of claims 17 to 19wherein the cetane improver is present in the fuel oil in a proportionin the range of from 5 to 10,000 ppm of active ingredient by weightbased on the weight of the fuel oil.
 21. The composition of claim 20wherein the proportion is from 50 to 2000 ppm.
 22. The composition ofclaim 10 wherein the or one of the co-additives is a polymer of a C₂ toC₆ mono-olefin, the polymer having a number average molecular weight ofless than about
 500. 23. The composition of claim 22 wherein the polymeris polyisobutylene.
 24. A method for operating a diesel engine to reduceone or more of particulate emissions, hydrocarbon emissions, carbonmonoxide emissions, and oxides of nitrogen emissions in operation of theengine, which method comprises combusting in the engine a compositioncomprising a major proportion of a fuel oil and a minor proportion of anadditive comprising an oil-soluble stable free radical or a precursortherefor that is convertible to an oil-soluble free radical under saidcombusting, the minor proportion of additive being such that it isincapable of increasing the cetane number of the fuel oil.